Tory Brexiteers came up with a 140-page proposal for leaving the European Union, which reportedly contained ideas for an elaborate missile defence system and an expeditionary force to defend the Falkland Islands.

The document, developed by the European Research Group (ERG), made up of Eurosceptic Tory backbenchers, came in response to the prime minister’s Chequers plan.

Ms May has challenged Brexiteers to come up with an alternative to her blueprint. However ERG chairman Mr Rees-Mogg told The Financial Times that the group had “reconsidered” publishing the full document, prompting speculation of divisions within the pro-Brexit wing of the party.

Mr Rees-Mogg did not answer questions on the full document when repeatedly pressed on it at a parliamentary event on the economic benefits of a no-deal Brexit – instead saying the ERG would unveil specific plans for the Northern Ireland border on Wednesday.

At the event, the campaign group Economists for Free Trade argued there could be substantial economic gains if the UK leaves the EU without a trade deal in its own report, backed by a string of senior Brexiteers.

Boris Johnson, whose marital breakdown has been making headlines, broke cover to attend the event, where he claimed Ms May’s plans were “substantially worse than the status quo”.

He avoided questions from journalists as he tried to leave the event, on whether he would be willing to oust Ms May from No 10 and also refused to answer if he had full confidence in the prime minister.

Asked about the prospects of Ms May facing a leadership challenge, he said: “This is absolutely nothing to do with that, this is all about trying to change the position on the European Union negotiations, which at the moment I think are verging on the humiliating for this country.”

Brexit-backing economists argued that a no-deal Brexit could boost Treasury coffers by £80bn over the next 15 years – claims that were branded “Project Fantasy” by critics.

Mr Rees-Mogg told the event: “What is the best way to achieve a deal? Is it to be snivelling and fearful and timorous and give the impression to the EU that we will do whatever they say?

“That we will kowtow, we will go down on bended knee, we will offer to serve homage to the great power that is in the European Union?

“Or do we go out and say [that] our best economists… say we have nothing to be fearful about?”

The audience, which included ex-Brexit secretary David Davis and a string of Eurosceptic Tories, applauded when he added: “We have nothing to fear but fear itself.”

Responding to the report, pro-EU MP Chuka Umunna said: “The latest Project Fantasy document from advocates of an ultra-hard Brexit promises to take us to a land of milk and honey, when the reality is that, before we have even left, Brexit is forcing up prices in the shops, destabilising our economy and stripping our health service of the doctors and nurses that it so badly needs.

“Whether it is Theresa May’s car crash Brexit, or Jacob Rees-Mogg’s disastrous no-deal Brexit, one thing we know about Brexit is that it is an unholy mess and the gap between what we were promised and the best we can hope for is getting wider every day.”